Structure and nature of the metal-support interface: characterization of iridium clusters on magnesium oxide by extended x-ray absorption fine structure spectroscopy

X-ray absorption spectroscopy was used to characterize the metal-support interface in catalysts consisting of very small Ir clusters of nearly uniform nuclearity on the surface of MgO powder. [Ir4(CO)12] on MgO was converted in high yield into [HIr4(CO)11]- and sep. into [Ir6(CO)15]2-. EXAFS data are consistent with the hypothesis that each cluster retained its nuclearity upon decarbonylation. The av. Ir-Ir coordination in Ir4/MgO is 2.6 neighbors at a distance of 2.72 .ANG., and the structure is modeled as a mixt. of 40-50% tetrahedra and the remainder 4-atom rafts on MgO. The av. Ir-Ir coordination in Ir6/MgO is 2.7 neighbors at a distance of 2.72 .ANG., and the structure is modeled as a mixt. of .apprx.25% Ir6 octahedra and the remainder Ir rafts on MgO. Another sample was treated in H2 at 623 K to give a typical supported metal catalyst having a distribution of Ir clusters averaging 10 .ANG. in diam. The Ir-support interfaces in Ir4/MgO and Ir6/MgO are characterized by single Ir-Mg coordinations at 1.6 .ANG. and 4-fold Ir-O coordinations at 2.6 .ANG.; the interface Ir atoms probably are atop Mg ions in the (100) faces of MgO. However, this structural model is simplified, and the Ir-Mg interactions are not fully understood. The structurally simple supported Ir clusters are among the simplest supported metals; the metal-O distances of .apprx.2.6 .ANG. are quite generally characteristic of metal particles and clusters supported on metal oxides and zeolites; the long distance is attributed to metal atoms interacting with O of the support with H present at the interface. [on SciFinder (R)